Method for improving acoustic model, computer for improving acoustic model and computer program thereof

ABSTRACT

Embodiments include methods and systems for improving an acoustic model. Aspects include acquiring a first standard deviation value by calculating standard deviation of a feature from first training data and acquiring a second standard deviation value by calculating standard deviation of a feature from second training data acquired in a different environment from an environment of the first training data. Aspects also include creating a feature adapted to an environment where the first training data is recorded, by multiplying the feature acquired from the second training data by a ratio obtained by dividing the first standard deviation value by the second standard deviation value. Aspects further include reconstructing an acoustic model constructed using training data acquired in the same environment as the environment of the first training data using the feature adapted to the environment where the first training data is recorded.

BACKGROUND

The present invention relates to a field of speech recognition, moreparticularly, to a technique for improving an acoustic model used inspeech recognition.

In speech recognition, a statistical method is used. The statisticalmethod is a method in which features of speech are accumulated fromtraining data which is a recorded large amount of speech data, and aword sequence which is the closest to the features is output as arecognition result while comparing input speech signals with theaccumulated features. Typically, acoustic features of the speech areoften dealt with separately from linguistic features. The acousticfeatures represent what kind of frequency property each phoneme of arecognition target has, and are referred to as an acoustic model (AM).

One technique for converting a cepstrum so as to match an acousticmodel, feature space maximum likelihood linear regression (hereinafter,also referred to as “FMLLR”) is known. The cepstrum which is a featurewidely used in speech recognition, is a signal obtained by performinginverse fast Fourier transform (IFFT) on a logarithmic power spectrum ofan observation signal for separating a signal of fine frequency property(for example, vocal cord vibration) from a signal which is an observedsignal that has passed through a filter having smooth frequency property(for example, a vocal tract).

BRIEF SUMMARY

Embodiments include methods, systems and computer program products forimproving an acoustic model. Aspects include acquiring a first standarddeviation value by calculating standard deviation of a first featurefrom a first training data acquired in first environment and acquiring asecond standard deviation value by calculating standard deviation of asecond feature from second training data acquired in a secondenvironment. Aspects also include calculating a modified first feature,by multiplying the second feature acquired from the second training databy a ratio obtained by dividing the first standard deviation value bythe second standard deviation value and reconstructing, by the computer,an acoustic model constructed using training data acquired in the firstenvironment, using the modified first feature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram illustrating an example of a computer used inaspects of the present invention or a computer according to the aspectsof the present invention;

FIG. 1B is a diagram illustrating an example of the computer used in theaspects of the present invention or the computer according to theaspects of the present invention in a case where one or a plurality ofvirtual machines are made to operate on the computer;

FIG. 2A is a flowchart for processing of improving an acoustic modelaccording to the aspects of the present invention;

FIG. 2B is a flowchart for processing of improving an acoustic modelaccording to the aspects of the present invention;

FIG. 2C is a flowchart for processing of improving an acoustic modelaccording to the aspects of the present invention;

FIG. 2D is a flowchart for processing of improving an acoustic modelaccording to the aspects of the present invention;

FIG. 3A is a diagram for processing of improving an acoustic model of acall center according to the aspects of the present invention;

FIG. 3B illustrates a character error rate in a case where the acousticmodel of the call center is improved and the improved acoustic model isused, according to the aspects of the present invention;

FIG. 4 illustrates an example of a functional block diagram of acomputer which preferably has a hardware structure according to FIG. 1Aor FIG. 1B and which implements the aspects of the present invention;and

FIG. 5 illustrates that when large field data is not available, there isacoustic mismatch between the large field data and the in-house dataeven using the in-house data in prior art.

DETAILED DESCRIPTION

Embodiments of the present invention will be described below accordingto the drawings. In the following description according to the drawings,the same reference numerals indicate the same objects unless there isnotice. It should be appreciated that the embodiments of the presentinvention are provided for explaining preferred embodiments of thepresent invention, and are not intended to limit the scope of thepresent invention.

FIG. 1A illustrates an example of a computer which can be used in theaspects of the present invention or a computer according to the aspectsof the present invention. While the computer can be, for example, one ora plurality of computers, for example, a server computer (for example, acomputer provided with a server function), the computer is not limitedto these.

A computer (101) includes one or a plurality of CPUs (102) and a mainmemory (103), which are connected to a bus (104). The CPU (102) is, forexample, based on architecture of 32 bits or 64 bits. The CPU (102) canbe, for example, Power™ series of International Business MachinesCorporation, Xeon® series, Core™ i series, Core™ two series, Pentium®series, Celeron® series, or Atom™ series of Intel Corporation, orOpteron™ series, A series, Phenom™ series, Athlon™ series, Turion®series, or Sempron™ of AMD (Advanced Micro Devices), Inc.

A display (106), for example, a liquid crystal display (LCD) can beconnected to the bus (104) via a display controller (105). Further, theliquid crystal display (LCD) may be, for example, a touch panel displayor a floating touch display. The display (106) can be used to display anobject which can be displayed through operation of software running onthe computer (101) (for example, a computer program according to theaspects of the present invention or arbitrary various computer programsrunning on the computer (101)), using appropriate graphic interface.Further, the display (106) can output a screen of, for example, webbrowser application.

Arbitrarily, a disk (108), for example, a hard disk or a solid statedrive (SSD) can be connected to the bus (104) via, for example, a SATAor an IDE controller (107). Arbitrarily, a drive (109), for example, aCD, a DVD or a BD drive can be connected to the bus (104) via, forexample, the SATA or the IDE controller (107). Arbitrarily, a keyboard(111) and a mouse (112) can be connected to the bus (104) via aperipheral device controller (110), via, for example, a keyboard mousecontroller or a USB bus.

In the disk (108), an operating system, for example, an operating systemdeveloped for a main frame (for example, z/OS, z/VM or z/VSE), Windows®,UNIX®, Linux®, MacOS®, and Android®, and a Java® processing environmentsuch as J2EE, Java® application, a Java® virtual machine (VM), a programproviding a (JIT) compiler upon execution of Java®, a computer programaccording to the aspects of the present invention, and other arbitraryvarious computer programs, and data can be stored so as to be able to beloaded to the main memory (103).

Further, in the disk (108), software which enables processing ofimproving an acoustic model according to the aspects of the presentinvention can be stored so as to be able to be loaded to the main memory(103). The disk (108) may be incorporated into the computer (101), orthe computer (101) may be connected via a cable so as to be able toaccess the disk (108), or the computer (101) may be connected via awired or wireless network so as to be able to access the disk (108). Thedrive (109) can be used as necessary for installing a program, forexample, an operating system, an application program, or the computerprogram according to the aspects of the present invention from a CD-ROM,a DVD-ROM, or a BD in the disk (108).

The communication interface (114) conforms to, for example, an Ethernet®protocol. The communication interface (114) is connected to the bus(104) via the communication controller (113), and plays a role ofconnecting the computer (101) to the communication line (115) in a wiredor wireless manner, and provides a network interface layer to a TCP/IPcommunication protocol of a communication function of the operatingsystem of the computer (101). It should be noted that the communicationline can be, for example, a wireless LAN environment based on wirelessLAN connection standard, a Wi-Fi wireless LAN environment such as IEEE802.11a/b/g/n, or a mobile phone network environment (for example, a 3G,LTE, or 4G environment).

FIG. 1B illustrates an example of a computer which can be used in theaspects of the present invention or a computer according to the aspectsof the present invention in a case where one or a plurality of virtualmachines are made to operate on the computer. The computer can beconfigured as, for example, a server computer such as a work station, arack-mount server, a blade server, a midrange and a main frame.

The computer (121) illustrated in FIG. 1B can include one or a pluralityof CPUs (131), a main memory (132), a storage (133), a communicationcontroller (134), and a communication interface (135) as hardwareresources (122). The above-described one or a plurality of CPUs (131),the main memory (132), the storage (133), the communication controller(134), the communication interface (135), and the communication line(136) can respectively correspond to one or a plurality of CPUs (102),the main memory (103), the disk (108), the communication controller(113), and the communication interface (114) of the computer (101), andthe communication line (115) illustrated in FIG. 1A.

Further, the computer (121) operates as a physical host machine, and canactivate one or a plurality of virtual machines 1 to n (125-1 to 125-2)(also referred to as domain U or child partitions) which use the same ordifferent OS (for example, Windows®, UNIX®, Linux®) as a guest OS (156)on a hypervisor (also referred to as a virtual monitor or a virtual OS)of virtual software (for example, VMWare®, Hyper-V®, Xen®).

Further, the computer (121) can activate a management virtual machine(124) (also referred to as domain 0 or a parent partition) on thehypervisor. The management virtual machine (124) includes a managementOS (141), a control module (142) operating on the management OS (141),and a virtual resource (143). The control module (142) is a module whichissues a command to the hypervisor (123). Further, the control module(142) issues a command for creating the virtual machines 1 to n (125-1to 125-2) of a user domain and activating the guest OS (156) to thehypervisor (123) to control operation of the virtual machines 1 to n(125-1 to 125-2). The virtual resource (143) is a hardware resource(122) assigned for the management virtual machine (124).

The virtual machines 1 to n (125-1 to 125-2) include the virtualresource, the guest OS (156) and various application 1 to n (157-1 to157-3) operating on the guest OS (156). The virtual resource includes,for example, a virtual CPU (151), a virtual memory (152), a virtual disk(153), a virtual communication controller (154), and a virtualcommunication interface (155).

The following FIG. 2A to FIG. 2D respectively illustrate flowcharts forprocessing of improving an acoustic model according to the aspects ofthe present invention.

The above-described acoustic model is an acoustic model created in thepast, and is an acoustic model constructed in the past using trainingdata acquired in the same environment as an environment of firsttraining data which will be described below. The acoustic model is atarget acoustic model for which accuracy is improved according to theaspects of the present invention. In the aspects of the presentinvention, there is a case where the acoustic model is referred to as atarget domain acoustic model.

The first training data is speech data acquired under the sameconditions as those of an operation environment, and can be, forexample, speech data recorded under the same conditions as those of anoperation environment where an acoustic model is used for speechrecognition. The first training data can be, for example, part of fielddata (in other words, small field data) acquired in the operationenvironment where the acoustic model is used for speech recognition.

The second training data is speech data acquired in a differentenvironment from an environment where the first training data isrecorded, and thus, speech data acquired under different conditions fromconditions of the above-described operation environment. The environmentdifferent from the environment of the first training data can be, forexample, an environment different from the environment where the firsttraining data is recorded (for example, location or recording equipment(including hardware and software for recording)). Further, the secondtraining data can be, for example, speech data recorded under differentconditions from those of the operation environment where the acousticmodel is used for speech recognition. The second training data can be,for example, in-house data acquired in a different environment from theoperation environment where the acoustic model is used for speechrecognition.

The amount of the first training data is smaller than the amount of thesecond training data. For example, when the amount of the first trainingdata is data corresponding to approximately 30 minutes, the amount ofthe second training data can be data corresponding to 150 minutes (itshould be noted that each figure is merely an example, and the aspectsof the present invention are not limited to these). Therefore, the firsttraining data can be data which is difficult to be acquired for a reasonof, for example, security, and the second training data can be datawhich can be easily acquired.

While the feature is, for example, a cepstrum or log mel filter bankoutput, the feature is not limited to these. The feature in a cepstrumdomain can include, for example, a mel frequency cepstrum coefficient(MFCC) and PLP (perceptual linear predictive). The ceptstrum and the logmel filter bank output are respectively known to a person skilled in theart. The computer (101) can use, for example, an arbitrarily programmedmodule to execute processing of the cepstrum or the log mel filter bankoutput.

FIG. 2A illustrates a flowchart for processing of improving an acousticmodel without using FMLLR according to the aspects of the presentinvention. As shown at block 201, the computer (101) starts processingof improving an acoustic model. As shown at block 202, the computer(101) reads out first training data (for example, small field data) froma recording medium (291) in which the first training data is stored,calculates a feature from the first training data, and calculatesstandard deviation of the feature to acquire a first standard deviationvalue. While small field data is required as the first training data, ifthe feature is a cepstrum, since a dimension of the cepstrum is low (forexample, approximately 13 dimensions), a very small amount of data isrequired to obtain the standard deviation.

As shown at block 203, the computer (101) reads out second training data(for example, in-house data) from a recording medium (292) in which thesecond training data is stored, calculates a feature from the secondtraining data, and calculates standard deviation of the feature toacquire a second standard deviation value. It should be noted that thecomputer (101) may execute the processing of block 202 and theprocessing of block 203 in parallel on, for example, different virtualenvironments, or may execute the processing of block 202 prior to theprocessing of block 203, or may execute the processing of block 203prior to the processing of block 202.

As shown at block 205, the computer (101) multiplies the featureacquired from the second training data as shown at block 203 by theratio obtained by dividing the first standard deviation value acquiredas shown at block 202 by the second standard deviation value acquired asshown at block 203 to create a feature adapted to the environment wherethe first training data is recorded.

As shown at block 207, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from a recording medium (293) inwhich the acoustic model is stored, and reconstructs the acoustic modelread out from the recording medium (293) using the feature created asshown at block 205. The computer (101) can store the reconstructedacoustic model in a storage medium (294) in which the reconstructedacoustic model is stored. In this manner, it is possible to acquire anacoustic model for which accuracy is improved by using the acousticmodel read out from the recording medium (293) as an initial value,converting the feature so that distribution of the second training datamatches distribution of the first training data, and reconstructing theacoustic model using the converted feature. The reconstructed acousticmodel is utilized for speech recognition in the same environment as theenvironment of the first training data. As shown at block 208, thecomputer (101) finishes processing of improving an acoustic model.

FIG. 2B illustrates a flowchart for processing of improving an acousticmodel further using FMLLR after a feature adapted to the environmentwhere the first training data is recorded is created according to theaspects of the present invention. As shown at block 211, the computer(101) starts processing of improving an acoustic model. Blocks 212, 213and 215 respectively correspond to blocks 202, 203 and 205 in FIG. 2A.Therefore, please refer to explanation of each of blocks 202, 203 and205 for explanation of each of blocks 212, 213 and 215.

As shown at block 216, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from the recording medium (293)in which the acoustic model is stored, and creates a feature by applyingFMLLR to the feature created as shown at block 215 using the acousticmodel. The feature is generated by applying the FMLLR to the feature,for example, as follows. First, when there are an acoustic model and agiven feature sequence, the computer (101) calculates an FMLLRconversion matrix which reduces mismatch of the environment between theacoustic model and the feature sequence. Then, the computer (101)generates a feature for which there is small mismatch of the environmentbetween the acoustic model and the feature sequence by multiplying thefeature sequence by the FMLLR conversion matrix.

As shown at block 217, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from the recording medium (293)in which the acoustic model is stored, and reconstructs the acousticmodel read out from the recording medium (293) using the feature createdas shown at block 216. The computer (101) can store the reconstructedacoustic model in a storage medium (294) in which the reconstructedacoustic model is stored. In this manner, it is possible to acquire anacoustic model for which accuracy is improved by using the acousticmodel read out from the recording medium (293) as an initial value,converting the feature so that distribution of the second training datamatches distribution of the first training data, and further estimatingan FMLLR conversion matrix for the acoustic model so as to match theconverted feature, generating a feature by multiplying the convertedfeature by the FMLLR conversion matrix, and reconstructing the acousticmodel using the created feature. The reconstructed acoustic model isutilized for speech recognition in the same environment as theenvironment of the first training data. As shown at block 218, thecomputer (101) finishes the processing of improving an acoustic model.

FIG. 2C illustrates a flowchart for processing of improving an acousticmodel further using FMLLR prior to creation of the feature adapted tothe environment where the first training data is recorded, according tothe aspects of the present invention. As shown at block 221, thecomputer (101) starts processing of improving an acoustic model. Blocks222 and 223 respectively correspond to blocks 202 and 203 in FIG. 2A.Therefore, please refer to explanation of each of blocks 202 and 203 forexplanation of each of blocks 222 and 223.

As shown at block 224, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from the recording medium (293)in which the acoustic model is stored, and creates a feature by applyingFMLLR to the feature created as shown at block 223 using the acousticmodel.

As shown at block 225, the computer (101) creates a feature adapted tothe environment where the first training data is recorded, bymultiplying the feature created as shown at block 224 by the ratioobtained by dividing the first standard deviation value acquired asshown at block 222 by the second standard deviation value acquired asshown at block 223.

As shown at block 227, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from the recording medium (293)in which the acoustic model is stored, and reconstructs the acousticmodel read out from the recording medium (293) using the feature createdas shown at block 225. The computer (101) can store the reconstructedacoustic model in the storage medium (294) in which the reconstructedacoustic model is stored. In this manner, it is possible to acquire anacoustic model for which accuracy is improved by using the acousticmodel read out from the recording medium (293) as an initial value,applying FMLLR to the feature using the acoustic model, and then,converting the feature so that distribution of the feature after theFMLLR is applied to the second training data matches distribution of thefirst training data, and reconstructing the acoustic model using theconverted feature. The reconstructed acoustic model is utilized forspeech recognition in the same environment as the environment of thefirst training data. As shown at block 228, the computer (101) finishesthe processing of improving an acoustic model.

FIG. 2D illustrates a flowchart for processing of improving an acousticmodel further using FMLLR both prior to and after creation of thefeature adapted to the environment where the first training data isrecorded, according to the aspects of the present invention. As shown atblock 231, the computer (101) starts the processing of improving anacoustic model. Blocks 232 and 233 respectively correspond to blocks 202and 203 in FIG. 2A. Therefore, please refer to explanation of each ofblocks 202 and 203 for explanation of each of blocks 232 and 233.

As shown at block 234, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from the recording medium (293)in which the acoustic model is stored, and creates a first feature byapplying FMLLR to the feature acquired as shown at block 223 using theacoustic model.

As shown at block 235, the computer (101) creates a feature adapted tothe environment where the first training data is recorded, bymultiplying the first feature created as shown at block 234 by the ratioobtained by dividing the first standard deviation value acquired asshown at block 222 by the second standard deviation value acquired asshown at block 223.

As shown at block 236, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from the recording medium (293)in which the acoustic model is stored, and creates a second feature byapplying FMLLR to the feature created as shown at block 235 using theacoustic model.

As shown at block 237, the computer (101) reads out an acoustic modelconstructed using training data acquired in the same environment as theenvironment of the first training data from the recording medium (293)in which the acoustic model is stored, and reconstructs the acousticmodel read out from the recording medium (293) using the second featurecreated as shown at block 236. The computer (101) can store thereconstructed acoustic model in the storage medium (294) in which thereconstructed acoustic model is stored. In this manner, it is possibleto acquire an acoustic model for which accuracy is improved by using theacoustic model read out from the recording medium (293) as an initialvalue, applying FMLLR to the feature using the acoustic model, and,then, converting the feature so that distribution of the first featureafter the FMLLR is applied to the second training data matchesdistribution of the first training data, further estimating an FMLLRconversion matrix for the acoustic model so as to match the convertedfeature, generating a feature by multiplying the converted feature bythe FMLLR conversion matrix and reconstructing the acoustic model usingthe created second feature. The reconstructed acoustic model is utilizedfor speech recognition in the same environment as the environment of thefirst training data. As shown at block 238, the computer (101) finishesthe processing of improving an acoustic model.

FIG. 3A and FIG. 3B illustrate diagrams for explaining processing ofimproving an acoustic model at a call center, according to the aspectsof the present invention. FIG. 3A illustrates a diagram for processingof improving an acoustic model at the call center, according to theaspects of the present invention. The call center (301) possesses largefield data (390). Further, the call center (301) possesses an acousticmodel (hereinafter, referred to as a “target domain acoustic model”)(393) constructed from the large field data (390). However, the targetdomain acoustic model (393) is constructed using an old technique at thetime of installation of the call center. Therefore, at the call center,it is tried to perform maintenance work for improving the target domainacoustic model (393) using the latest technique as one of measures forimproving speech recognition.

An operating company of the call center (301) tries to outsource themaintenance work to an external company. However, the large field data(390) partly includes confidential information (personal information ofpersonal who is a client), and thus cannot be provided to the externalcompany. Therefore, the operating company of the call center (301)extracts part which does not include confidential information from thelarge field data (390) and provides the part (hereinafter, referred toas “small field data”) (for example, data corresponding to approximately30 minutes) (391) to the external company.

The external company tries to perform the following maintenance work forimproving the target domain acoustic model (393) using the latesttechnique using a server computer (301). Further, the external companypossesses available in-house data (for example, data corresponding toapproximately 150 minutes) (392) which is required for the abovemaintenance work of the acoustic model.

As shown at block 312, the server computer (301) calculates a cepstrumfrom the small field data (391) and calculates standard deviationσ_field of the cepstrum to acquire a first standard deviation value. Asshown at block 313, the server computer (301) calculates a cepstrum fromthe in-house data (392) and calculates standard deviation σ_in-house ofthe cepstrum to acquire a second standard deviation value. As shown atblock 315, the server computer (301) multiplies the cepstrum of thein-house data (392) by σ_field/σ_in-house to create a feature adapted tothe environment where the small field data (391) is recorded. As shownat block 316, the server computer (301) creates a feature by applyingFMLLR to the feature created as shown at block 315 using the targetdomain acoustic model (393).

As shown at block 317, when FMLLR is not applied, the server computer(301) reconstructs the target domain acoustic model (393) using thefeature created as shown at block 315, or when FMLLR is applied, theserver computer (301) reconstructs the target domain acoustic model(393) using the feature created as shown at block 316. The computer(101) stores the reconstructed acoustic model in a storage medium as animproved acoustic model (394). The external company provides theimproved acoustic model (394) through a storage medium which can beutilized by the operating company of the call center (301). Theoperating company of the call center (301) uses the improved acousticmodel (394) for speech recognition at the call center (that is, in thesame environment as the environment where the large field data isrecorded).

FIG. 3B illustrates a character error rate when the target domainacoustic model (393) of the call center is improved according to theaspects of the present invention and the improved acoustic model isused, as an example of the present invention, and a character error ratewhen the target domain acoustic model (393) of the call centerconstructed in the past is used as is or when the target domain acousticmodel (393) is improved only using FMLLR according to the priortechnique, as a comparison example.

For each of the target domain acoustic models (393) respectivelyobtained from the call center A and the call center B, the followingexperiments of the examples and the comparison examples were performedto improve the target domain acoustic model (393) constructed in thepast from the large field data (390).

Example 1 corresponds to the processing of the flowchart illustrated inFIG. 2A. In Example 1, an acoustic model was improved by, for each ofthe target domain acoustic models (393) respectively obtained from thecall center A and the call center B, among the diagram illustrated inFIG. 3A, omitting processing of block 316 (that is, in a case where theFMLLR is not applied), and in reconstruction of the target domainacoustic model (393) as shown at block 317, reconstructing the targetdomain acoustic model (393) using the feature created as shown at block315. Speech recognition was respectively performed at the call center Aand the call center B using the improved acoustic models, and thecharacter error rates (CER) were calculated.

The above-described character error rate which is recognition rateassessment measure and a method in which matching is performed in unitsof character, is known to a person skilled in the art.

Example 2 corresponds to the processing of the flowchart illustrated inFIG. 2B. In Example 2, an acoustic model was improved by, for each ofthe target domain acoustic models (393) respectively obtained from thecall center A and the call center B, among the diagram illustrated inFIG. 3A, executing the processing of block 316 (that is, in a case wherethe FMLLR is applied), and in reconstruction of the target domainacoustic model (393) of block 317, reconstructing the target domainacoustic model (393) using the feature created as shown at block 316.Speech recognition was performed at each of the call center A and thecall center B using the improved acoustic models, and the charactererror rates were calculated.

In Comparison Example 1, speech recognition was performed at each of thecall center A and the call center B using the target domain acousticmodels (393) respectively obtained from the call center A and the callcenter B, and the character error rates were calculated.

In Comparison Example 2, an acoustic model was improved by using thetarget domain acoustic models (393) respectively obtained from the callcenter A and the call center B, and, among the diagram illustrated inFIG. 3A, omitting blocks 312 to 315, and in the processing of block 316,creating the feature by applying FMLLR to the cepstrum of the in-housedata (392) using the target domain acoustic model (393), and, inreconstruction of the target domain acoustic model (393) of block 317,reconstructing the target domain acoustic model (393) using the featurecreated from the in-house data. Speech recognition was performed at eachof the call center A and the call center B using the improved acousticmodels, and the character error rates were calculated.

By the way, in the reconstruction of the acoustic model, processing isperformed in order from (1) estimation of a maximum likelihood of HiddenMarkov Model (HMM)—Gaussian Mixture Model (GMM), (2) discriminationtraining of a feature region, and (3) discrimination training in a modelregion.

At the call center A, for the target domain acoustic model (393), theabove-described (1) estimation of a maximum likelihood of Hidden MarkovModel—Gaussian Mixture Model and (2) discrimination training of afeature region have been already performed, and (3) discriminationtraining in a model region was performed using the feature according tothe aspects of the present invention. It should be noted that inComparison Example 2 for the call center A, the processing of theabove-described (3) was performed using the feature created by applyingFMLLR without using the aspects of the present invention.

At the call center B, for the target domain acoustic model (393), theabove-described (1) estimation of a maximum likelihood of Hidden MarkovModel—Gaussian Mixture Model has been already performed, and (2)discrimination training of a feature region and (3) discriminationtraining in a model region were performed using the feature according tothe aspects of the present invention. It should be noted that inComparison Example 2 for the call center B, the processing of theabove-described (2) and (3) was performed using the feature created byapplying FMLLR without using the aspects of the present invention.

For each of the call center A and the call center B, results ofcharacter error rates in Examples 1 and 2 and Comparison Examples 1 and2 are illustrated in a table of FIG. 3B. In a case of the call center A,the character error rate was 0.2% improved in Example 1 compared toComparison Example 1. Further, the character error rate was 0.6%improved in Example 2 compared to Comparison Example 1. Still further,the character error rate was 0.3% improved in Example 2 compared toComparison Example 2.

In a case of the call center B, the character error rate was 5.2%improved in Example 1 compared to Comparison Example 1. Further, thecharacter error rate was 6.0% improved in Example 2 compared toComparison Example 1. It should be noted that in Comparison Example 2,even if a training method is changed in various manners, the charactererror rate does not fall below 54.0.

FIG. 4 illustrates an example of a functional block diagram of acomputer which preferably has a hardware structure according to FIG. 1Aor FIG. 1B and which implements the aspects of the present inventionaccording to the flowcharts illustrated in FIG. 2A to FIG. 2D. Acomputer (401) can correspond to the computer (101) illustrated in FIG.1A or the computer (121) illustrated in FIG. 1B. The computer (401)includes a standard deviation value calculating unit (411), a featurecreating unit (412) and an acoustic model reconstructing unit (413).Further, the computer (401) can arbitrarily include an FMLLR applyingunit (414). The standard deviation value calculating unit (411) readsout first training data from a recording medium (491) in which the firsttraining data is stored, calculates a feature from the first trainingdata, and calculates standard deviation of the feature to acquire afirst standard deviation value, and reads out second training dataacquired in a different environment from an environment of the firsttraining data from a recording medium (492) in which the second trainingdata is stored, calculates a feature from the second training data, andcalculates standard deviation of the feature to acquire a secondstandard deviation value.

Further, the standard deviation value calculating unit (411) can executeblocks 202 and 203 illustrated in FIG. 2A, blocks 212 and 213illustrated in FIG. 2B, blocks 222 and 223 illustrated in FIG. 2C andblocks 232 and 233 illustrated in FIG. 2D. Further, the feature creatingunit (412) creates a feature adapted to an environment where the firsttraining data is recorded, by multiplying the feature acquired from thesecond training data or the feature created by applying FMLLR to thefeature acquired from the second training data using the target domainacoustic model (493) by the ratio obtained by dividing the firststandard deviation value by the second standard deviation value.Further, the feature creating unit (412) can execute block 205illustrated in FIG. 2A, block 215 illustrated in FIG. 2B, block 225illustrated in FIG. 2C and block 235 illustrated in FIG. 2D. Theacoustic model reconstructing unit (413) reconstructs the acoustic model(that is, the target domain acoustic model (493)) constructed usingtraining data acquired in the same environment as the environment of thefirst training data, using the feature adapted to the environment wherethe first training data is recorded.

Further, the acoustic model reconstructing unit (413) can execute block207 illustrated in FIG. 2A, block 217 illustrated in FIG. 2B, block 227illustrated in FIG. 2C and block 237 illustrated in FIG. 2D. The FMLLRapplying unit (414) creates a feature by applying FMLLR to the featurecreated by the feature creating unit (412) or the feature acquired fromthe second training data (492) at the standard deviation valuecalculating unit (411), using the target domain acoustic model (493).Further, the FMLLR applying unit (414) can execute block 216 illustratedin FIG. 2B, block 224 illustrated in FIG. 2C and blocks 234 and 236illustrated in FIG. 2D.

As illustrated in FIG. 5, when a speech recognition system isconstructed in a given call center, generally, a target domain acousticmodel (503) is constructed using large field data (501) recorded at thecall center (504). It should be noted that the field data is speech datarecorded under the same environment conditions as those at the time whenthe acoustic models are actually operated. However, in a stage of theabove-described maintenance work, the large field data is not alwaysavailable (501), because the field data includes confidentialinformation (for example, personal information), and there is a casewhere the field data cannot be utilized for a reason of security, or thelike.

Further, as available data, there is data (for example, in-house data(502)) which is separately recorded in an environment different from theabove-described call center. However, since there is acoustic mismatchbetween the in-house data (502) and the field data (for example, thelarge field data (501)) (511), accuracy of the already constructedacoustic model (503) cannot be expected to be improved merely by simplyutilizing the in-house data (502).

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

What is claimed is:
 1. A method for improving an acoustic model,comprising: acquiring, by a computer, a first standard deviation valueby calculating standard deviation of a first feature from a firsttraining data acquired in first environment; acquiring, by the computer,a second standard deviation value by calculating standard deviation of asecond feature from second training data acquired in a secondenvironment; calculating, by the computer, a modified first feature, bymultiplying the second feature acquired from the second training data bya ratio obtained by dividing the first standard deviation value by thesecond standard deviation value; and reconstructing, by the computer, anacoustic model constructed using training data acquired in the firstenvironment, using the modified first feature.
 2. The method accordingto claim 1, wherein environment includes the recording hardware andsoftware used to acquire training data, and wherein the feature is aceptsrum, and wherein the first training data is speech data recordedunder the same conditions as conditions of an operation environment, andthe second training data is speech data recorded under differentconditions from conditions of the operation environment.